wp-content/themes/twentyfifteen/inc/back-compat.php:52Prevents the Customizer from being loaded on WordPress versions prior to 4.1.
function twentyfifteen_customize() { wp_die( sprintf( /* translators: %s: WordPress version. */ __( 'Twenty Fifteen requires at least WordPress version 4.1. You are running version %s. Please upgrade and try again.', 'twentyfifteen' ), $GLOBALS['wp_version'] ), '', array( 'back_link' => true, ) );}Introduced in Twenty Fifteen 1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-content/themes/twentyfifteen/inc/back-compat.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.